This invention relates generally to heater arrays for an ink jet printer head, and more particularly to a heater array having combined resistor and diode heating elements.
A typical ink jet printer head contains an ink reservoir, in which the ink completely surrounds an internal heater array. The heater array typically contains multiple heating elements such as thin or thick film resistors, diodes, and/or transistors. The heating elements are arranged in a regular pattern for heating the ink to the boiling point. Each heating element in the heater array can be individually or multiply selected and energized in conjunction with other heating elements to heat the ink in various desired patterns, such as alpha-numeric characters. The boiled ink above the selected heating elements shoots through corresponding apertures in the ink jet printer head immediately above the heater array. The ink jet droplets are propelled onto printer paper where they are recorded in the desired pattern.
A schematic of a typical resistor type heater array is shown in FIG. 1. It should be noted that other types of heater arrays are used, wherein each resistor is individually addressed and coupled to a common ground node. Heater array 10, however, includes multiple row select lines A.sub.1 through A.sub.M, wherein select lines A.sub.1 through A.sub.3 are shown, and multiple column select lines B.sub.1 through B.sub.N, wherein select lines B.sub.1 through B.sub.3 are shown. Spanning the row and column select lines are resistor heating elements R.sub.11 through R.sub.MN, wherein resistor heating elements R.sub.11 through R.sub.33 are shown. A specific resistor is selected and energized by, for example, grounding a column line coupled to one end of the resistor and applying a voltage to the appropriate row line coupled to the opposite end of the resistor.
One problem with heater array 10 involves unwanted power dissipation due to "sneak paths." Such sneak paths energize resistor heating elements other than the one desired, even if non-selected row and column select lines are open-circuited. Sneak paths in heater array 10 are best demonstrated by analyzing the current flow in the array. If resistor R.sub.11 is selected a current flows between row select line A.sub.1 and column select line B.sub.1. However, a parallel resistive path exists through non-selected resistors R.sub.12, R.sub.22, and R.sub.21, even if row select line A.sub.2 and column select line B.sub.2 are both open-circuited. If row select line A.sub.1 is more positive than column select line B.sub.1, current flows through row select line A.sub.1 into resistor R.sub.12, through column select line B.sub.2, through resistor R.sub.22, through row select line A.sub.2, through resistor R.sub.21, and finally into column select line B.sub.1. This is but one example of numerous sneak paths in the heater array 10, involving every resistor in the array. Due to the undesirable sneak paths in heater array 10 and consequent energizing of nonselected heating elements, the power dissipation of the array is unnecessarily and significantly increased.
A schematic of a typical diode type heater array is shown in FIG. 2. Heater array 11 includes the same multiple row and column select lines shown in the resistor heater array 10. Spanning the row and column select lines are diode heating elements D.sub.11 through D.sub.MN, wherein diode heating elements D.sub.11 through D.sub.33 are shown. A specific diode heating element is selected and energized by, for example, grounding a column line coupled to the cathode of the diode and applying a current to the appropriate row line coupled to the anode of the diode.
The problem of sneak paths is substantially eliminated in heater array 11 due to the unidirectional current flow allowed by the diode heating elements. For example, if diode D.sub.11 is selected a current flows into row select line A.sub.1 through diode D.sub.11 and out of column select line B.sub.1. However, the sneak current flow path that existed in the resistive heater array 10 through non-selected resistors R.sub.12, R.sub.22, and R.sub.21, no longer exists. Current flowing out of the cathode of diode D.sub.11 cannot flow into the cathode of diode D.sub.21. Similarly, current flowing into the anode of diode D.sub.11 cannot flow into the anode of diode D.sub.12, since the cathode of diode D.sub.12 is coupled to the cathode of diode D.sub.22.
Although the problem of sneak paths is substantially solved in heater array 11, another problem exists regarding the physical layout of the diodes on an integrated circuit. Typically, discrete diodes are fabricated on a crystalline silicon substrate to form the array. Since each diode must be made physically large to handle a large current density necessary to boil the ink, and since each diode must be insulated from adjacent diodes, the resulting array occupies a large silicon die area. Consequently, the size and topography of the integrated heater array limits the maximum number of discrete ink jets that can be produced. Another problem with the diode array 11 is that the diodes are not current limited and therefore the power dissipation of the array can be excessive. Still another problem is that the array is fabricated using an expensive integrated circuit process.
A combination transistor/resistor array 12 is shown in FIG. 3. Again, the row and column select lines are identical to those shown in arrays 10 and 11. Spanning the row and column select lines are resistor heating elements R.sub.11 through R.sub.MN, wherein resistor heating elements R.sub.11 through R.sub.33 are shown, in series with field-effect transistors M.sub.11 through M.sub.MN, wherein transistors M.sub.11 through M.sub.33 are shown. In contrast to the previous heater arrays, the column select lines are coupled to and selectively energize the gates of the transistors. No heating current actually flows through the column select lines. The row select lines are typically coupled to a power supply voltage or a high impedance. The heating occurs in the resistors similar to array 10, with all the heating current flowing to ground and not from column line to row line.
The configuration of array 12 also solves the problem of sneak paths as well as unlimited power consumption, since the power is limited by the applied voltage at the row select lines and value of the heating resistors. However, as in array 11, the maximum size of the array is limited and the cost of the array is high due to the conventional integrated circuit fabrication techniques that are used. Similar problems exist in an integrated heater array using discrete resistors and diodes.
What is desired is a low cost, low power, and compact fabrication technique for an ink jet heater array.